The mushroom farm will be a 2X4 meters rectangle with 2 meters height.

I will be using this water pump: http://viewitem.eim.ebay.pt/200GPH-Aquarium-Air-Water-Pump-Fish-Tank-Submersible-Pond-Fountain-Powerhead-12WEU230V/310569179711/item?variant0=EU-230V&transId=337425859021 with the EU plug (230V)

I don't think you'd be able to hook that pump into the computer power supply that easily (perhaps if you ripped it open you'd find it runs on a 12V motor or something, but it'd be a lot easier to just plug it in regularly).

And well I don't know much about mushroom farming so some of this might be a little off. But I'm guessing you're gonna need to test the air humidity. The moisture content of your soil, and the temp.Then based off of those youll trigger the mister and the fans to keep the moisture and temp at the happy place correct?

to control the pump, you could easily look for something like thishttps://www.sparkfun.com/products/10747Only in a 230V version. Basically its a switch that you control with a digital pin from your arduino (trigger it on whenever the humidity gets below like 70%,then after the airs back to 85% or whatever humidity you want, trigger it off to turn off the pump)

Something like that would be extremely easy to use, but after peeking around for a little (i see your from portugal) I'm not finding a control box as easy to use as the 120V one I linked above.

Those are fairly easy to use. Basically you take an extension chord. Cut 1 of the 2 conductors, attaching each end of the now cut conductor to 1 of the 2 "output" terminals on that relay (make sure to do a nice hook around the screw). Then plug the remaining end into the wall (probably have someone electrically inclined to check your cable for safety).

Then connect an arduino digital pin to the + on the control side. and connect the - to the arduinos ground. now when you trigger the arduino pin it'll turn the pump on or off (assumign the pump is plugged into this modified extension chord)

Thatll let you control the misting. then you just need a temp sensor and your humidity sensor

Best way to start out would probably be grab some LEDs, get them to trigger on and off (these will emulate the relays). Then grab your sensors, get them to read data, and set up your humidity and temperature points so that when its too dry or whatever, itll turn on the LEDs.Then grab the pumps and relays, and just replace the LEDs with the relays, and your on your way to a nice thick shiitake stew.

I noticed you mentioned 2 relays though and the LCD, do you need a LCD to be recommended, of have you picked out one you like (because they can become problematic sometimes).Also whats the second relay for, to trigger the computer power supply off and on to run the fans? Or will there be 2 misting pumps?

For safety reasons, it would be nice if you could avoid mains voltages. You could switch the 12V going to each of the fans via transistors, even use PWM if you need variable speed. Same thing with the pump, although you'll need to find one powerful enough at 12V and a more beefy transistor that the fans take. The pumps that spray water at your windscreen must be 12V - powerful enough?

pressure does depend on the pipe you use... yes, but it depends mostly on the pump that will be driving it. Basically, the smaller diameter pipe you have, the higher pressure water you need to move the same volume of water through (just like higher resistance wire takes a higher voltage to drive the same current). Your 6mm pipe, with a low pressure pump will just slowly trickle out water, not in any misting form.

If instead you want a fine spray, you'd need a high pressure pump running through that small pipe (and then through some kind of misting nozzle) Kinda liek a windex bottle, I rmeember readign that those produce a fairly high pressure when you squeeze the trigger.

I'll get back to you about the sensors and LCD screen though, gotta get to work.